nmigen needs some very specific additions to support the LibreSOC layout. these include adding SRNOR Latches, as well as "z" and "x" in signals (migen supported "x" but nmigen does not).
ongoing maintenance of nmigen and associated libraries for five months * 200+ patches reviewed * https://gitlab.com/nmigen/nmigen/-/merge_requests * https://gitlab.com/nmigen/nmigen/-/issues * https://gitlab.com/nmigen/nmigen-boards